Flakes: builtin function to flatten inputs using follows

[FRidh]

Is your feature request related to a problem? Please describe.
Having to specify inputs.foo.inputs.bar.follows everywhere can become tedious. Sometimes it is needed because packages need to be upgraded in lockstep.

Describe the solution you'd like
A builtin function that, given a set of inputs, would set the follows of each input to the other inputs. Essentially, this function would flatten the inputs tree as shown by nix flake metadata as much as possible. Users can choose to apply the function on the entire set of inputs, or on one or more subsets.

Describe alternatives you've considered
While it is possible to implement the function in Nix directly, it is not possible to share such function (would require copying) or actually even use it, because Nix doesn't allow it.

Maybe a function likes this needs to take parameters to exclude certain inputs from being followed. I don't have this use case but I can imagine it.

Additional context
This may also be desirable as the whole dependency tree of a flake grows, and there will be many "copies" of Nixpkgs and/or other flakes.

[gytis-ivaskevicius]

I'd be down with an alternative as shown in the picture.

Currently, let/in syntax is not allowed in a flake.nix file. Not sure why that is the case but there probably is some sort of reasoning behind it :/

[kanashimia]

Currently, let/in syntax is not allowed in a flake.nix file. Not sure why that is the case but there probably is some sort of reasoning behind it :/

Reason: #3075
Related issue: #3966
It is a major annoyance indeed.

Basically having any function on the flake attributes is impossible right now, by design.

[DieracDelta]

I've also run into this problem a bunch and it is quite annoying to manually need to specify everywhere. I quite like the solution @gytis-ivaskevicius has proposed.

[gytis-ivaskevicius]

@DieracDelta, to implement it in the way that edolstra would be happy with - this would need to be done which is probably somewhat complex #3966 (comment), so yeah.. Don't put too much hope in this solution

[nrdxp]

I think, given the current implementation of flakes, it may be easier to implement this on the cli level. So maybe something like nix flake follows nixpkgs would search the dependencies of every flake recursively, and for any flake that has a nixpkgs input, set it to follow the root flake's nixpkgs.

[stale]

I marked this as stale due to inactivity. → More info